Skip WAL for new relfilenodes, under wal_level=minimal.

 Skip WAL for new relfilenodes, under wal_level=minimal.
 
 Until now, only selected bulk operations (e.g. COPY) did this.  If a
 given relfilenode received both a WAL-skipping COPY and a WAL-logged
 operation (e.g. INSERT), recovery could lose tuples from the COPY.  See
 src/backend/access/transam/README section "Skipping WAL for New
 RelFileNode" for the new coding rules.  Maintainers of table access
 methods should examine that section.
 
 To maintain data durability, just before commit, we choose between an
 fsync of the relfilenode and copying its contents to WAL.  A new GUC,
 wal_skip_threshold, guides that choice.  If this change slows a workload
 that creates small, permanent relfilenodes under wal_level=minimal, try
 adjusting wal_skip_threshold.  Users setting a timeout on COMMIT may
 need to adjust that timeout, and log_min_duration_statement analysis
 will reflect time consumption moving to COMMIT from commands like COPY.
 
 Internally, this requires a reliable determination of whether
 RollbackAndReleaseCurrentSubTransaction() would unlink a relation's
 current relfilenode.  Introduce rd_firstRelfilenodeSubid.  Amend the
 specification of rd_createSubid such that the field is zero when a new
 rel has an old rd_node.  Make relcache.c retain entries for certain
 dropped relations until end of transaction.
 
 Bump XLOG_PAGE_MAGIC, since this introduces XLOG_GIST_ASSIGN_LSN.
 Future servers accept older WAL, so this bump is discretionary.
 
 Kyotaro Horiguchi, reviewed (in earlier, similar versions) by Robert
 Haas.  Heikki Linnakangas and Michael Paquier implemented earlier
 designs that materially clarified the problem.  Reviewed, in earlier
 designs, by Andrew Dunstan, Andres Freund, Alvaro Herrera, Tom Lane,
 Fujii Masao, and Simon Riggs.  Reported by Martijn van Oosterhout.
 
 Discussion: https://postgr.es/m/20150702220524.GA9392@svana.org